1. Field of the Invention:
The present invention relates to a braking force holding device for a vehicle equipped with an air brake system, intended for facilitating the start of the vehicle on an upward hill.
2. Description of the Prior Art:
Japanese Laid-Open Patent Publication No. 58-105867, for instance, discloses a braking force holding device comprising a main body having an inlet port connected to a brake valve which discharges compressed air when operated by the driver, an outlet port connected to a brake actuator which is actuated by compressed air, and a passage interconnecting the inlet port and the outlet port, a first valve provided in the passage, capable of detecting the hill-climbing condition of the vehicle, and capable of closing upon the detection of the hill-climbing condition to intercept the flow of air from the outlet port to the inlet port, and a second valve provided in the passage in parallel to the first valve, having a valve element capable of being moved in the opening direction by a pressure applied thereto on the side of the inlet port and the resilience of a valve spring, and capable of being moved in the closing direction by a liquid pressure applied thereto by a clutch actuator to intercept the flow of air from the outlet port to the inlet port.
In operation, the first valve closes when the vehicle comes to a hill. Then, when the clutch actuator is operated to disengage the clutch after supplying compressed air to the brake actuator by operating the brake valve to stop the vehicle, liquid pressure produced by the clutch actuator shifts the valve element of the second valve against the pressure applied to the valve element on the side of the inlet port and the resilience of the valve spring to close the second valve. Thus, both the first and second valves are closed to disconnect the inlet port and the outlet port. Consequently, the pressure applied to the brake actuator is maintained even if the operation of the brake valve is cancelled after the vehicle has been braked to a stop as long as the clutch is held disengaged, so that the braking force is maintained to hold the vehicle stopped. During the starting of the vehicle, when the clutch engaging operation is performed in combination with the operation of the accelerator, the effect of the liquid pressure applied to the valve element by the clutch actuator decreases below the resilience of the valve spring to allow the valve element to be shifted in the opening direction and hence, the second valve is opened. Thus, the braking pressure maintained in the brake actuator is relieved as the driving force of the engine is transmitted through the clutch to the driving wheels. Accordingly, the vehicle can be easily started on a hill without requiring simultaneous dexterous manipulation of the hand brake, the accelelator and the clutch.
In this known braking force holding device, the second valve is opened to start relieving the brake holding pressure after the liquid pressure applied to the second valve by the clutch actuator has dropped to a fixed level below the resilience of the valve spring and the frictional engagement of the clutch has been enhanced to a fixed degree. Thus, the opening timing of the second valve is dependent on the resilience of the valve spring regardless of the level of the brake holding pressure. On the other hand, in an air brake system employing compressed air as a working fluid, the pressure reduction rate, namely, pressure reduction per unit time, is comparatively high at the moment of actuation of the brake pressure relief valve. However, the pressure reduction rate decreases with time. Therefore, the air brake system requires a comparatively long time, as compared with a hydraulic brake system, for sufficiently relieving the brake pressure. Such a characteristic of the air brake system is enhanced when the brake pressure is high.
Accordingly, when the grade of the hill or the weight of the vehicle is large and the brake pressure is maintained at a high level while the vehicle is stopped, the relief of the brake pressure is delayed relative to the degree of frictional engagement of the clutch according to the progress of clutch engaging operation. Therefore, the brake pressure is not reduced according to the increase of the driving force transmitted to the driving wheels through the clutch, and hence, an excessive braking force works on the vehicle against the driving force. Consequently, the drag of the brakes, the excessive slip of the clutch and engine stop are likely to occur.
Thus, the conventional braking force holding device causes the rapid abrasion of the clutch facing and the brake linings or impedes the smooth start of the vehicle, when the braking pressure is maintained at a high level.